For example, a plunging type constant velocity universal joint that is axially displaceable while forming an operating angle but forms a relatively small maximum operating angle is generally assembled on an inboard side (differential side) of an automotive front drive shaft. Further, a fixed type constant velocity universal joint that can form a large operating angle but is not axially displaceable is generally assembled on an outboard side (wheel side) of the automotive front drive shaft because the wheel is steered on the outboard side.
FIG. 23 illustrate a Rzeppa type constant velocity universal joint 101 as an example of the fixed type constant velocity universal joint that is used on the outboard side. FIG. 23a is a vertical sectional view of a state at an operating angle of 0°, and FIG. 23b is a schematic view of a state in which a maximum operating angle is formed. As illustrated in FIG. 23a, the constant velocity universal joint 101 mainly includes an outer joint member 102, an inner joint member 103, balls 104, and a cage 105. Eight track grooves 107 are formed equiangularly in a spherical inner peripheral surface 106 of the outer joint member 102 so as to extend along an axial direction. Track grooves 109 opposed to the track grooves 107 of the outer joint member 102 are formed equiangularly in a spherical outer peripheral surface 108 of the inner joint member 103 so as to extend along the axial direction. Eight balls 104 for transmitting torque are interposed between the track grooves 107 of the outer joint member 102 and the track grooves 109 of the inner joint member 103. The cage 105 for holding the balls 104 is arranged between the spherical inner peripheral surface 106 of the outer joint member 102 and the spherical outer peripheral surface 108 of the inner joint member 103. An outer periphery of the outer joint member 102 and an outer periphery of a shaft coupled to the inner joint member 103 are covered with a boot, and grease is sealed inside the joint as a lubricant (not shown).
As illustrated in FIG. 23a, the cage 105 has a spherical outer peripheral surface 112 fitted to the spherical inner peripheral surface 106 of the outer joint member 102, and a spherical inner peripheral surface 113 fitted to the spherical outer peripheral surface 108 of the inner joint member 103. The spherical outer peripheral surface 112 and the spherical inner peripheral surface 113 each have a curvature center formed at a joint center O. On the other hand, a curvature center Oo of a ball raceway center line x of each track groove 107 of the outer joint member 102 and a curvature center Oi of a ball raceway center line y of each track groove 109 of the inner joint member 103 are offset to both sides in the axial direction by equal distances with respect to the joint center O. Therefore, when the joint forms an operating angle, the balls 104 are always guided in a plane bisecting an angle formed between axial lines of the outer joint member 102 and the inner joint member 103. As a result, rotational torque is transmitted at a constant velocity between the two axes.
As illustrated in FIG. 23b, a maximum operating angle θmax, which is defined as a main function of the fixed type constant velocity universal joint 101, depends on an angle causing interference between an inlet chamfer 110 formed at an opening rim of the outer joint member 102 and a shaft 111. In order to secure permissible torque to be transmitted, an axial diameter d of the shaft 111 is determined for each joint size. When a large inlet chamfer 110 is formed, the length of each track groove 107 of the outer joint member 102, on which the ball 104 is brought into contact (hereinafter referred to as “effective track length”), is insufficient. As a result, the ball 104 may drop off the track groove 107, and the rotational torque cannot be transmitted. Therefore, how the inlet chamfer 110 is formed while securing the effective track length of the outer joint member 102 is an important factor in securing the operating angle. In the Rzeppa type constant velocity universal joint 101, the curvature center Oo of the ball raceway center line x of the track groove 107 of the outer joint member 102 is offset to an opening side. Thus, there is an advantage in terms of the maximum operating angle, and the maximum operating angle θmax is approximately 47°.
Further, as compared to a related-art constant velocity universal joint of a six ball type, the Rzeppa type constant velocity universal joint 101 of the eight ball type has a smaller track offset amount, a larger number of balls, and has a smaller diameter. Thus, it is possible to attain a highly efficient constant velocity universal joint that is lightweight and compact, and is suppressed in torque loss. However, as illustrated in FIG. 24, at an operating angle of 0°, wedge angles α formed between the opposed track grooves 107 and 109 of the outer joint member 102 and the inner joint member 103 (as illustrated in FIG. 24, the contact points between the ball 104 and the track grooves 107 and 109 are positioned on the broken lines) are opened toward the opening side of the outer joint member 102. Therefore, due to axial force W applied from the track grooves 107 and 109 to the balls 104, loads to be applied to the spherical contact portions 106 and 112 of the outer joint member 102 and the cage 105 and the spherical contact portions 108 and 113 of the inner joint member 103 and the cage 105 are generated in a certain direction. In this structure, as illustrated in FIG. 24, the outer joint member 102 and the cage 105 are held in contact with each other at a part J, and the inner joint member 103 and the cage 105 are held in contact with each other at a part I, which leads to restriction on achieving even higher efficiency and less heat generation.
In order to achieve even higher efficiency and less heat generation than those of the above-mentioned Rzeppa type constant velocity universal joint 101 of the eight ball type, a fixed type constant velocity universal joint of a track groove crossing type has been proposed ( JP 2009-250365 A). FIGS. 25 and 26 illustrate the constant velocity universal joint of this type. FIG. 25 is a vertical sectional view at an operating angle of 0°, and FIGS. 26 are views at a high operating angle. As illustrated in FIG. 25, a constant velocity universal joint 121 mainly includes an outer joint member 122, an inner joint member 123, balls 124, and a cage 125. Although illustration is omitted, in the constant velocity universal joint 121 of the track groove crossing type, planes including ball raceway center lines x of eight track grooves 127 of the outer joint member 122 are inclined with respect to a joint axial line n-n with their inclination directions opposite to each other in the track grooves 127 adjacent to each other in a peripheral direction. In addition, each track groove 129 of the inner joint member 123 has a ball raceway center line y, which is formed so as to be mirror-image symmetrical with the ball raceway center line x of the paired track groove 127 of the outer joint member 122 with respect to a plane P including a joint center O at the operating angle of 0°.
In the vertical cross section illustrated in FIG. 25, each track groove 127 formed in a spherical inner peripheral surface 126 of the outer joint member 122 extends into an arc shape along the axial direction, and a curvature center of the track groove 127 is positioned at the joint center O. Each track groove 129 formed in a spherical outer peripheral surface 128 of the inner joint member 123 so as to be opposed to the track groove 127 of the outer joint member 122 extends into an arc shape along the axial direction, and a curvature center of the track groove 129 is positioned at the joint center O. Eight balls 124 for transmitting torque are interposed in crossing portions between the track grooves 127 of the outer joint member 122 and the track grooves 129 of the inner joint member 123. The cage 125 for holding the balls 124 is arranged between the spherical inner peripheral surface 126 of the outer joint member 122 and the spherical outer peripheral surface 128 of the inner joint member 123. The cage 125 has a spherical outer peripheral surface 132 fitted to the spherical inner peripheral surface 126 of the outer joint member 122, and a spherical inner peripheral surface 133 fitted to the spherical outer peripheral surface 128 of the inner joint member 123. The spherical outer peripheral surface 132 and the spherical inner peripheral surface 133 each have a curvature center formed at the joint center O. In the constant velocity universal joint 121, curvature centers of the ball raceway center lines x and y of the track grooves 127 and 129 of the outer joint member 122 and the inner joint member 123 are not offset in the axial direction with respect to the joint center O. However, the inclined opposed track grooves 127 and 129 cross each other, and the balls 124 are interposed in those crossing portions. Therefore, when the joint forms an operating angle, the balls 124 are always guided in a plane bisecting an angle formed between axial lines of the outer joint member 122 and the inner joint member 123. As a result, rotational torque is transmitted at a constant velocity between the two axes.
In the above-mentioned fixed type constant velocity universal joint 121 of the track groove crossing type, the track grooves 127 of the outer joint member 122 that are adjacent to each other in the peripheral direction are inclined in the opposite directions. Further, the track grooves 129 of the inner joint member 123 that are adjacent to each other in the peripheral direction are inclined in the opposite directions. Therefore, forces in the opposite directions are applied from the balls 124 to pocket portions 125a of the cage 125 that are adjacent to each other in the peripheral direction. Due to the forces in the opposite directions, the cage 125 is stabilized at the position of the joint center O. Thus, a contact force between the spherical outer peripheral surface 132 of the cage 125 and the spherical inner peripheral surface 126 of the outer joint member 122, and a contact force between the spherical inner peripheral surface 133 of the cage 125 and the spherical outer peripheral surface 128 of the inner joint member 123 are suppressed. Accordingly, the joint is smoothly operated under high load and in high speed rotation. As a result, torque loss and heat generation are suppressed, and the durability is enhanced.
The above-mentioned fixed type constant velocity universal joint 121 is excellent as a joint suppressed in heat generation, but has the following problem inherent therein. Detailed description is given of the problem with reference to FIG. 26. FIG. 26a illustrates a state in which the above-mentioned constant velocity universal joint forms a high operating angle, and FIG. 26b illustrates a positional relationship between the track groove 127 of the outer joint member 122 and the ball 124 on an enlarged scale. As illustrated in FIG. 26a, when the joint forms a high operating angle θ, a center Ob of the ball 124 moves to a position of θ/2 with respect to the plane P including the joint center O at the operating angle of 0°. The ball 124 and the track groove 127 are held in angular contact at a contact angle, and hence a contact point between the ball 124 and the track groove 127 is positioned on the broken line in FIG. 26b. In addition, in the axial direction, the contact point between the ball 124 and the track groove 127 is positioned in a plane t that passes through the center Ob of the ball 124 and is orthogonal to the ball raceway center line x. In the above-mentioned fixed type constant velocity universal joint 121, when a large inlet chamfer 130 is formed in the outer joint member 122, the ball 124 comes to a position on an outside of the inlet chamfer 130 at a high operating angle θ, and may drop off the track groove 127. This is because the effective track length becomes insufficient. Specifically, the curvature center of the arc-shaped track groove 127 and the joint center O match with each other, and hence an axial distance w between the center Ob of the ball 124 and a contact point s is large. As a result, there arises a problem that high operating angles cannot be formed.